Cellulose is one of the major components in plant cell wall and is also a major resource of biomass on earth. Hence, many enzymes that degrade cellulose can be widely applied in many different industries. Cellulose is a polysaccharide composed of glucose units linked by β-1,4-glycosidic bond. These polysaccharides organize tightly together to form crystalline cellulose in order to defense destructing energy from outside of plant. On the other hand, many kinds of herbivores and microbes need to degrade cellulose from plant to glucose as an energy source by different degrading enzymes including cellulase, xylanase and so on. The catalytic mechanism of cellulase involves hydrolyzing the β-1,4-glycosidic bond between two sugar units by acid-base interaction. Cellulase can be generally divided into three groups including endoglucanase, cellobiohydrolase and β-glucosidase. Endoglucanase can randomly degrade cellulose into many small fragments. Cellobiohydrolase can degrade cellulose from reducing end or non-reducing end to release main product, cellobiose. β-Glucosidase can degrade cellobiose into simple sugar glucose.
So far, the industrial applications of cellulase are widespread in food industry, feed industry or textile industry, even in biofuel production. In general, cellulase needs to conform to different appropriate conditions according to different industrial needs. For example, acidic and thermostable enzymes are suitable for the feed industry but textile industry prefers alkaline enzymes. Therefore, scientists always try to seek better enzymes which are more suitable for different industrial needs in academic or industrial researches. Currently, many researchers and enzyme companies could produce better enzymes by screening in nature or modifying present enzymes. There are generally two strategies of enzyme modification including directed evolution that randomly mutates the enzyme gene and selects with desirable properties and rationale engineering that specifically mutates the enzyme gene based on the structural information of the enzyme.
Therefore, the present invention intends to analyze the enzyme structure of the cellulase for finding out the key amino acid important to the enzymatic activity and further modify the enzyme, so as to improve the enzymatic activity of the cellulase and thus increase the industrial value of the cellulase.